In a telecommunications network, digital private branch exchanges (PBXs) are interconnected by pairs of modulator-demodulator (mod-demod) machines for use in transmitting facsimile (fax) signals. The mod-demod machines provide for a savings in bandwidth and thus transmission cost by demodulating a fax signal down to a baseband level before passing it across connections in the network and by subsequently remodulating the signal for transmission to a PBX. In particular, one mod-demod machine of each tandem pair demodulates a 64 k bits per second (bps) pulse code modulation (PCM) signal down to a baseband level for transmission to other mod-demod machines in the network. The other mod-demod machine of the pair remodulates the baseband signal back to a 64 k bps PCM signal for subsequent transmission in the network.
Unfortunately, modulation and demodulation introduce distortion and delay into a signal, and when a signal is subjected to repeated modulation and demodulation cycles, each cycle adds distortion and delay. Repeated modulation and demodulation of a fax signal often results when a connection established within a telecommunications network includes more than one PBX. The increased amount of distortion resulting from repeated modulation and demodulation is undesirable, as it may result in a loss of data or errors in transmitted data.
Accordingly, a need exists for an improved method of transmitting fax signals in a digital communications network.